1. Field of the Invention
the present invention relates to temperature controlled ovens, particularly ovens employed in the stages of fabricating micro-electronic semi-conductor devices and the like.
2. Description of the Prior Art
In the production of solid-state micro-electronic devices, such as multi-layered LSI circuit chips, it is necessary to repetitively subject work in process to elevated, constant temperature. Such solid state devices include circuitry which is becoming smaller and more complex as time goes on. As the circuitry is miniaturized, flaws introduced during manufacture become more problematic. In order to avoid flaws caused by contaminants during the heat treatment of these devices, it is important that the heat treatment environment be substantially free of possible contaminants.
Contaminants in the heated atmosphere substantially decrease the yield obtained in producing substrates for integrated circuits, microprocessors, LSI circuit arrays and the like. Many ordinary commercial ovens are known to produce in excess of 50,000 particles 0.5 microns or larger in a cubic foot of air. For the processing of solid state electronic devices, it is desired to reduce the particle level within a processing oven to not more than 100 particles 0.5 microns or larger per cubic foot of air. Elimination of particles below this standard has not been reliably achieved by conventional processing ovens, particularly for high temperature applications.
Contamination is generated by the heating elements, and particularly ceramic disks used to support the heating elements, within a heat controlled oven. Additionally, the silicon wafers or other chip substrates may themselves contaminate the oven chamber. The fiberglass insulation of the oven enclosure also produces some contamination. External contamination has been introduced to the oven from the blower and the blower motor.
Prior efforts to construct heat treating ovens for such purposes have centered on making the oven chambers readily cleanable. Rounded corners have been provided and care has been exercised in the use and selection of materials within the oven. In addition, in heated air ovens of the type disclosed in U.S. Pat. No. 4,460,332, a removable second subassembly with an air filter is provided in an attempt to satisfy class 100 air purification standards.
Frequently, heat treatment of electronic substrates requires processing within inert atmospheres in order to minimize oxidation of silicon and metallized layers which may be present, as the possibility of oxidation increases with higher temperature. In such cases, a special inert atmosphere oven is required that is constructed so as to minimize leakage of air into the oven or leakage of inert atmosphere out of the oven through the door seal, fan motor shaft seal, seams, or any penetration through oven walls. Such an oven is typified by the Despatch Industries Model LND 1-42 inert atmosphere bench oven. In such ovens, inert gas, such as nitrogen, is typically fed into the oven chamber within which silicon wafers have been placed. Flow of the filtered nitrogen is typically controlled by purge and maintain flowmeters. A three-way valve is usually supplied to select the purge or maintain flow levels.
The nitrogen in the Model LND 1-42 oven and similar inert atmosphere ovens is typically recirculated through a HEPA filter to remove particles from the oven environment. This helps eliminate both particles introduced in the nitrogen supplied to the oven, which is typically unfiltered, and particles which may be generated by the workpiece itself. However, this type of oven suffers from an inherent functional temperature limitation of about 220.degree.0 C. due to the use of the HEPA filters in recirculating nitrogen within the oven. Although some HEPA filters can be used above 220.degree. C., for a brief period of time, prolonged or frequent use of the filters at such temperatures will tend to cause the binders used in the filters to degrade. When these filters are then heated or cooled, they will tend to shed particles, making the filters themselves a source of the particles they are intended to remove.
One could use prefiltered nitrogen to pass into the furnace and pass the nitrogen through the oven only once, leaving the HEPA filter in a cooler environment to eliminate shedding. However, this solution tends to be too expensive to use on a commercial basis for at least two reasons. First, such single-pass operation will require a constant, relatively high volume supply of fresh nitrogen. As such nitrogen tends to be more expensive than other, more conventional gases (e.g. air), this is frequently cost prohibitive to use on a commercial scale. Additionally, the nitrogen will all have to be heated to the desired oven temperature or above, increasing fuel costs as compared to a recirculating system wherein heated gas is retained in the oven. Hence, both ovens using in-line HEPA filters, such as the Model LND 1-42 oven, and single-pass ovens have inherent limitations which prevent them from being used in a commercially effective manner for high temperature heat treatment when an inert atmosphere is necessary.
In addition to the use of an inert gas atmosphere, it is necessary to insure that the elevated temperatures within the heated chamber are relatively constant and do not vary by more than a stated number of degrees between any two points in the chamber. Close temperature uniformity throughout an oven chamber makes processing of any product more reliable. In general, product consistency is improved by increasing the amount of inert gas recirculation around the work in process as this will tend to minimize temperature variations in the chamber. Because uniformity is temperature dependant, product variability generally increases as oven temperature increases. Thus, it is also necessary to insure that a temperature controller operates accurately and quickly in response to temperature changes detected by thermocouples situated within the chamber to reduce variability from one product run to the next and to minimize localized temperature variations within the chamber during a single run.
Typically, an inert gas, such as nitrogen, is filtered through a class 1 or better filter as it is recirculated within the enclosure or chamber to contact the work in process. In the despatch industries Model LND 1-42 inert atmosphere bench oven, forced convected heat is employed as described above. Forced convection utilizes a fan to create gas flow which supplies heat more effectively to all parts of the chamber. The addition of forced air flow represents a significant improvement in overall temperature uniformity and in the time to transfer heat to the work in process. Air directed against a product heats it up much faster then merely surrounding the product with heat and prevents stratifications and other localized temperature variations sometimes found in gravity ovens.
Recirculating air flow, on the other hand, recreates a specific air distribution pattern throughout the chamber that depends on the inlet and outlet locations, the size of the chamber, the positioning of baffles, the air flow output of the fan, and other factors. This pattern can itself introduce some temperature variations within the chamber. Recirculating air or inert gas flow may also be disadvantageous when employed in the processing of integrated circuits as it may recirculate contaminants over the substrates.
Moreover, at higher temperatures (e.g. above about 300.degree. C.), radiant heat transfer becomes of greater significance in bringing the temperature of work in process to the radiant heating element temperature. Radiant heating, advantageously, may provide temperature uniformity if the radiant heat is uniformly emitted from all points surrounding the work in process. It has been difficult to achieve such uniformity of emitted radiant heat in conventional heat treatment ovens, though, as radiant heat can be dependent upon the geometry of the oven and the relative position of the workpiece within the oven.
Many micro-electronic devices, including multi-layered LSI circuit chips and the like, are particularly sensitive to the presence of oxygen when the device is being processed. For many such devices, the presence of even a minimal amount of oxygen within the oven in which it is being heated can oxidize a sufficient portion of the work in progress to yield an unacceptable final device. In the past, a positive pressure of heated inert gas was maintained within the chamber to prevent the influx of ambient air into the chamber. Such an extensive use of nitrogen or other inert gas, however, will tend to increase fabrication costs for the devices as such inert gases are more expensive than other, more common gases, such as air.